Your memory may quick fill up. Its recommended to remove the video from the phone after you have uploaded it in the group.
Its recommended to change a low video resolution, otherwise the video will be heavy to upload and will take long . Typical video resolutions in smartphones are HD and FULL HD, both are way too heavy, your video file will be close to 1GB with those resolutions. Recommended resolutions are either VGA (640x480) or QVGA (320x240) QVGA being the most lightweight (5min video fits in 50MB file)
While you probably record the video with your selfie camera, it is useful to notice where the camera lense is located. Usually it is located above the screen. Your eyes will be seen better in the video, if you watch straight to the lense instead of watching the phone screen.
If you record in a standing position - for example outdoor, a selfie stick may become handy
You may want to check your mirroring settings in your video/camera options. In some phones selfie camera produces mirrored image by default.
It is also possible to record a live video directly in the Facebook app. In that case, the Facebook app mirrors the image even when you don't have mirroring selected in your camera/video settings.